Why do the devs continue to add ugly killers?

Remember that the trickster is primarily an appeal to the Korean market for mor business. What we find appealing and what they do might not be the same.

Plague is a great example, because from a distance, she is tall, elegant, and beautiful, while still being ominous and unapproachable. The closer you look, though, the more grotesque she is, with the charred flesh, rotting body, and writhing maggots. It's a multi-layered approach that reflects her backstory beautifully and gives her nuance. A top-notch job from the DBD artists.

This is opposed to, say, Twins, who look like sad, frumpy peasants from far away and are just slightly dirtier, frumpy peasants up close. I get that they were going for a Kuato look from Total Recall, but totally missed the mark. The little guy isn't especially deformed or malevolent-looking, the wound he lives in isn't especially gooey or bloody, and his big sister just looks like some woman. You can even see her regular-ass eyes! No darkness or glow or unnatural hue; she looks just like a heavy-set survivor! Surely somebody over there knows about the "eyes are the windows to the soul" theme, because look at every single other killer in the game. I can't imagine how Twins made it past the brainstorming stage.

Twins are uniquely unqualified to be killers, both in appearance and lore (and, some might say, gameplay mechanics). If you're going to go pretty-but-disturbing, looking at Trickster is a fine target. You can even say Meyers and Pyramid Head, if you're thirsty for Big Daddies. If you're going hideous and revolting, Hillbilly's closer to the target, with his deformed upper body and inhuman proportions. All killers certainly shouldn't look alike, but they really ought to have some kind of frightful aspect to them; a brooding hostility or seething malevolence. "Pretty" or "Ugly" aside, the killers have to be COOL.

I mean-- some of the killers are kinda, oh, I don't know, dead 😐

The entity isn't trying to pick celebrities, Trickster doesn't really really fit with the horror aesthetic, when I am being chased by him, I feel like a boy band reject is trying to kill me cuz I didn't like their album.

The devs did good with Legion and Ghostface, they are good for human characters. I also enjoy the designs of Nurse, Spirit, Trapper, Hag, and pretty much all the killers. Each are unique in their own way, as for killers like Nemesis, it's a license, they don't have permission to change the design, plus Nemesis is iconic ^^